Non-Patent Document 1 discloses an AlGaN/GaN high electron mobility transistor having a recess gate structure. The epitaxial layer composed of AlGaN/GaN is formed on the (0001) plane of a sapphire substrate by metal organic chemical vapor deposition. The epitaxial layer includes a GaN nucleation layer having a thickness of 20 nm, an undoped GaN layer having a thickness of 2.5 μm, an Al0.26Ga0.74N barrier layer, an n+-Al0.26Ga0.74N layer having a thickness of 20 nm, and an n+-GaN layer having a thickness of 20 nm. The flow rates of ammonia and triethygallium in order to grow the undoped GaN layer are 5 liters/minute and 69 μmol/minute, respectively. In order to grow the AlGaN layer, the flow rates of ammonia, triethygallium and trimethyl aluminum are 5 liters/minute, 29.5 μmol/minute, and 5.2 μmol/minute, respectively.    Non-Patent Document 1: T. Egawa, et al. Appl. Phys. Lett., Vol. 76, No. 1, pp. 121-123, 3 Jan. 2000.